That is metallic sodium that would keep in kerosene so it doesn't react with the oxygen in the air.

金属钠应该保存在没有中,这样它就不会和空气中的氧气反应了。

麻省理工公开课 - 固态化学导论课程节选

And we haven't talked about reactions at all yet, so you don't need to worry about the specifics of that exactly, but just that if you have this negative change in energy, you have a more stable product than you do reactant.

当然我们还没有开始讨论反应，所以你不用担心它的细节是怎么样的，你只需要知道，如果能量的变化是负值，那么你将得到一个比反应物更稳定的生成物。

麻省理工公开课 - 化学原理课程节选